WW2010 University of Illinois

 WW2010

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 Online Guides

 introduction

 > meteorology

 remote sensing

 projects, activities

 Meteorology

 introduction

 air masses, fronts

 clouds, precipitation

 el nino

 > forces, winds

 hurricanes

 hydrologic cycle

 light, optics

 midlatitude cyclones

 severe storms

 weather forecasting

 Forces, Winds

 introduction

 > pressure

 coriolis force

 geostrophic wind

 friction

 boundary layer wind

 sea breezes

 land breezes

 Pressure

 > definition

 variation with height

 isobars

 pressure surfaces

 variation with temps

 high pressure center

 low pressure center

 User Interface

 graphics
 > text

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Atmospheric Pressure
force exerted by the weight of the air

Atmospheric pressure is defined as the force per unit area exerted against a surface by the weight of the air above that surface. In the diagram below, the pressure at point "X" increases as the weight of the air above it increases. The same can be said about decreasing pressure, where the pressure at point "X" decreases if the weight of the air above it also decreases.

Thinking in terms of air molecules, if the number of air molecules above a surface increases, there are more molecules to exert a force on that surface and consequently, the pressure increases. The opposite is also true, where a reduction in the number of air molecules above a surface will result in a decrease in pressure. Atmospheric pressure is measured with an instrument called a "barometer", which is why atmospheric pressure is also referred to as barometric pressure.

 In aviation and television weather reports, pressure is given in inches of mercury ("Hg), while meteorologists use millibars (mb), the unit of pressure found on weather maps.

As an example, consider a "unit area" of 1 square inch. At sea level, the weight of the air above this unit area would (on average) weigh 14.7 pounds! That means pressure applied by this air on the unit area would be 14.7 pounds per square inch. Meteorologists use a metric unit for pressure called a millibar and the average pressure at sea level is 1013.25 millibars.

Forces, Winds
 Terms for using data resources. CD-ROM available. Credits and Acknowledgments for WW2010. Department of Atmospheric Sciences (DAS) atthe University of Illinois at Urbana-Champaign.

variation with height